Fire alarm with an aerosol indicator for detecting combustion products suspended in the air



Oct. 27, 1964 E. MEILI ETAL 3,154,773

FIRE ALARM WITH AN AEROSOL INDICATOR FOR DETECTING COMBUSTION PRODUCTSSUSPENDED IN THE AIR Filed Feb. 23, 1961 B am omw ATTORNEY United StatesPatent 3,154,773 FIRE ALARM WITH Al i AETR$GL INDlCATGR FDR BETECTlNG(ZDfviifildT-CEON SUSPENDED 1N AER Ernst l'vl'eili, Kusnaeht, and ThomasRampart, Mlannedorf, witzerland, assignors to ijerherns AG., Mannedorf,Switzerland, a corporation of Switzerland Filed 23, 1951, Ser. No.9%,989 Claims priority, application Switzerland, Feh. 2d, 396%, 2,208/605 Claims. (Cl. 343-237) The present invention relates to a novel firealarm system of the type provided with an aerosol indicator fordetecting combustion products suspended in the air. For the purposes ofthe present disclosure, the term combustion products relates toparticles of microscopic and submicroscopic sizes which are produced bycombustion and which float or are suspended in the A number of firealarm systems are known which respond to these particles and, morespecifically, aerosols in order to actuate, for example, a fire alarm.By way of example, mention is made of ionization fire alarms whichrespond to visible and invisible combustion products. Also, alarms whichoperate with a light source and a photosensitive cell which respondeither to a decrease in the intensity of the light received by thephotosentive cell from the light source in the event that aerosols arepresent between them, or to the refiection of light by the aerosols.

The heretofore mentioned indicators of aerosols provided in fire alarmsystems operate prefectly if the air does not contain substantialportions of other suspended contaminants, such as dust, soot, vapors andthe like. If such contaminants are present, the operation of the firealarm systems is adversely affected, or rendered impossible, since thecontaminants cause the same efiect as the combustion products which areto be detected. The contaminants may have the same effect on thetransparency or reflective power or" a gas at the aerosols, and they mayalter the current flowing through an ionization chamber in the samemanner as the aerosols stemming from combustion. In rooms andsurroundings in which dust or other impurities are present, thesensitivity or" the indicators must be considerably reduced if continualfalse alarms are to be avoided. Obviously, such an alteration in thesystem always involves a loss of sensitivity in detection of actualfires.

A further detrimental efiect experienced with the operation of firealarm systems provided with aerosol indicators is due to the soiling ofthese indicators by dust. if a breakdown after a short period ofoperation is to be prevented, costly cleaning processes are required.

Accordingly, the present invention has for one of its primary objects toprovide a fire alarm system of the type mentioned which obviates thedisadvantages mentioned hereinabove, which operates reliably even inrooms containing heavily contaminated air, and which responds mostsensitively to the aerosols formed by combustion, even if such aerosolsare present in very low concentrations.

Another important object of the present invention is to provide meanspreventing contaminants, other than those generally resulting from acombustion process, for actuating a fire alarm system.

Still a further object of the present invention is to provide means forfiltering out foreign particles within a predetermined size range priorto entry into a fire alarm system in order to preserve the integrity ofsaid system.

Yet another object of the present invention is to provide means forimproving the sensitivity of a fire alarm sys- 3,154,773 Patented Oct.27, 1964 tern in detecting the presence of combustion products, such asaerosols.

Extensive tests have revealed the surprising result that the particlesto which the aerosol indicators of the aforementioned type respond arecommonly smaller than one ,u, while the particles usually contained inair, by way of example, soot or dust particles, are considerably larger.The symbol ,u. as employed herein is to be understood as designatingmicrons.

According to the teachings of this invention the disadvantageousinfluence of the foreign particles frequently present in air is overcomeby causing the air to first pass through a filter before it reaches theindicators. The fire alarm according to the present invention ischaracterized by the fact that a filter is provided through which thegas to be tested reaches the aerosol indicator under the influence of apressure difference created across said filter. The filter is preferablydesigned in such a manner that it will allow at least the majority ofthe aerosol particles having a diameter smaller than one ,u to pass,while it retains the majority of particles having a diameter of morethan five It has been found that a fire may be reliably indicated bymeans of aerosol indicators even in Very dusty or contaminated premisesif these preconditions are met.

Still further objects of the present invention and the entire scope ofapplicability thereof will become apparent from the detailed desciptiongiven hereinafter; it should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

In the drawing:

FEGURE 1 illustrates a fire alarm system provided with a light sourceand a photoelectric cell between which the air to be tested is passed;

FIGURE 2 shows an ionization fire alarm in which the air to be testedconsecutively passes through a filter in both directions;

FIGURE 3 shows an ionization fire alarm arranged, by way of example, ina conduit or line; and

FIGURE 4 shows several fire alarms connected to a common fan whichproduces the pressure difference at the filters.

Referring now to FIGURE 1, the fire alarm system of the presentinvention is provided with an indicator comprising a light source 3 anda photoelectric cell 4 to which a microvoltmeter 5 may be connected. Thelight source 3 and the photoelectric cell 4 are located at diametricallyopposite locations of a tube or housing 6 in which a fan 2 is located.One end of the housing defining an inlet is covered by a filter 1. Inoperation, the fan 2 draws the air to be tested through the filter 1 andinto the housing 6 in the direction of the arrows shown.

The normal dust particles are retained by the filter 1 so that theycannot impede the light passing from the light source 3 to thephotoelectric cell However, if combustion products occur, aerosolparticles of which the diameter is less than about one ,u. will bepresent in the indicating area of the housing 6. These particles willpass through the filter l and cause the light energy passing to thephotoelectric cell 4- to be reduced. This results in a reduction of thevoltage which will be indicated by the microvoltrneter 5. Such reductionof voltage is an indication of the presence of a combustion process. Themicrovoltrneter 5 may also be replaced by relay members or the like,which are suitable to trigger the alarm if the voltage supplied by thephotoelectric cell 4 drops below a predetermined value.

The filter 1 may be formed of a nylon or cotton fabric, much in the samemanner as presently employed as a filtering material in commercialvacuum cleaners. For the present application, filters formed ofnon-swelling fibers may be advantageous, since the filtering propertieswill then be independent of humidity. In particular, filters formed ofspun-glass fabrics may be found to be suitable. As hereinbefore stated,the filters are designed so that, for example, particles having adiameter or greatest di- 'rnension preferably less than one ,u. arepassed, While particles of a dimension exceeding five U. are retained.The fiber grade employed in each particular case will also depend on thedegree of contamination of the air.

It may here be pointed out that the filter 1 will also retain part ofthe particles produced in combustion. However, experience has shown thatthe smaller particles having a diameter less than about one u willeasily sufiice to ensure reliability of indication of a combustionprocess.

In FIGURE 2 there is shown an ionization fire alarm system in which thegas to be tested is capable of passing through the filter 11 in bothdirections. The indicator of the fire alarm, according to FIGURE 2,contains two electrodes 14 and 15 connected to an indicating ormeasuring instrument 17 provided with a direct voltage source 18.Provided for one electrode, such as electrode 15, is a radioactivematerial 16, which at least partly ionizes the space between the twoelectrodes 14 and 15. If aerosols reach the space between theelectrodes, the ions, as is generally known, will adhere to theaerosols, thus reducing the effective mobility of the oins. This willresult in a certain decrease in the ionization current. The currentdecrease may be detected by the sensitive current indicator 17 or beemployed to initiate an alarm by means of a relay member. Furtherdetails of the ionization fire alarm system can best be ascertained byreferring to the copending US. application of Heinrich Derfier, SerialNo. 710,732, filed I an. 23, 1958, now United States Patent 2,994,768,issued August 1, 1961, and commonly as signed.

The two electrodes 14 and 15 are enclosed by a filter 11 which isdesigned and formed in the same manner as disclosed in conjunction withFIGURE 1. Enclosed in the space encompassed by the filter 11 is ahousing or cylinder 19 within which a piston 13 reciprocates. The piston13 may be connected, by way of example, with a connecting rod 20eccentrically attached to a crank or wheel 12 which may be driven by asynchronous motor. In operation, the piston 13 reciprocates in thecylinder 19 and, thus, causes air to be consecutively drawn in andforced out of the housing 19. In the suction stroke, as indicated by thearrows in FIGURE 2, air from the outside is drawn through the filter 11into the space between the two electrodes 14 and 15, whereas at leastportions of the air located in this space are subsequently passed to theoutside of the housing through the filter 11 during the pressure stroke.

In the embodiment according to FIGURE 2, which may naturally be combinedwith an optical aerosol indicator, the filter 11 cannot become soiledsince the dirt particles which are deposited on the outside surface ofthe filter during the suction stroke are blown away during the pressurestroke. The filter will, therefore, remain operative for considerableperiods without necessitating servicing.

FIGURE 3 shows a further embodiment of an ionization fire alarm. Theelectrical members are arranged in a manner similar to FIGURE 2, so thatfor clarity in comprehension the same reference numerals have beenemployed. The two electrodes of the indicator are arranged within atubular housing 23 which may be arranged, for example, in a duct orconduit. The gas to be tested for combustion products flows through theduct in the direction indicated by the arrows. Provided at the side ofthe housing 25 which faces the incoming air stream is a filter 21 whichretains the aerosol particles having a diameter of, for example, overfive ,u. while the particles having a smaller diameter are allowed topass. In the embodiment according to FIG. 3, the filter 21, furthermore,performs the function of reducing the flow within the tube 23. This isdesirable because excessive flow may change the relationship between theelectrodes 14 and 15 so as to reduce indicator sensitivity.

it may be appreciated from the examples disclosed that, either anexisting flow or, a pressure difference is employed to pass the air tobe tested through the filter, or means may be provided to produce such apressuredifierence (FIGURES 1 and 2).

If means for artificially producing a pressure difference on the twosides of a filter must be provided, it will be possible to connectseveral individual alarms to a common negative-pressure source, as forexample to a fan. Such an arrangement is shown in FIGURE 4. Three firealarms A, B and C, each comprising an aerosol indicator 32 and a filter31, are connected, via lines 34, with a fan 33 which maintains constantnegative pressure in the line 34. This negative pressure will regularlycause air to be drawn through the filters 31 and tested in the indicator32. Soiling of the filters 31 may be avoided by forcing air through thelines 34 in the op posite direction at regular intervals for the purposeof cleaning the filters 31. As generally known in the art, theindicators 34 are connected with a central station which may actuate analarm if the content of floating particles or aerosols having a diameterof, for example, one t exceeds a predetermined critical value.

It is readily to be appreciated that the pressure difierence in theembodiments shown may influence the response threshold of the alarm. Byway of example, if the pressure difference in the alarm is relativelygreat, a larger number of floating particles will pass through thefilter than if the pressure ditlerence is smaller. Reliable monitoringand communication, respectively, may therefore require control of thepressure difference at the filters for the reason just mentioned.

Having thus described the present invention, what is desired to besecured by United States Letters Patent is:

1. The combination with a fire alarm, for detecting the presence ofproducts of combustion contained in a fluid medium, of the typecomprising circuit means for operating said fire alarm, housing meanshaving an inlet opening and internally provided with an ionizationchamber communicating with said inlet opening and including spacedelectrode means and radioactive material provided in said ionizationchamber for at least partially ionizing the space therebetween, filtermeans operatively associated with said inlet opening of said housing forpreventing particles entering said ionization chamber whichsubstantially result from other than a combustion process, said filtermeans being constructed to pass particles having a diameter smaller than1 micron while retaining the majority of particles having a diametergreater than 5 microns.

2. The combination of claim 1, including a plurality of fire alarms,each provided with at least one separate ionization chamber, andindividual filter means operatively associated with said ionizationchamber of each fire alarm for preventing said particles from enteringeach ionization chamber which result from other than a combinationprocess.

3. The combination of claim 1, including means cooperating with saidfilter means for creating a pressure dilference across said filtermeans.

4. The combination of claim 3, said pressure diiference creating meansbeing a pump, said pump drawing, via said filter means, fluid mediuminto said housing during its suction stroke and forcing at least aportion of said fluid medium, via said filter means, from said housingduring 5 6 its exhaust stroke, to carry out a cleaning action on saidsaid duct member such that a pressure difierence is crefilter means.ated across said filter means.

5. The combination of claim 1, wherein said housing means is anelongated member provided with openings References sited 111 the file Ofthis Patent at opposite ends, said filter means being arranged in one 5UNITED STATES PATENTS of said ends to contact fluid medium flowing inthe direction of said one end, with said housing means adapted X222 x2to be arranged within a duct member, said housing means 2,572,008Cahusac et a1 Oct. 23 1951 possessing a cross-sectional area withrespect to that of

1. THE COMBINATION WITH A FIRE ALARM, FOR DETECTING THE PRESENCE OFPRODUCTS OF COMBUSTION CONTAINED IN A FLUID MEDIUM, OF THE TYPECOMPRISING CIRCUIT MEANS FOR OPERATING SAID FIRE ALARM, HOUSING MEANSHAVING AN INLET OPENING AND INTERNALLY PROVIDED WITH AN IONIZATONCHAMBER COMMUNICATING WITH SAID INLET OPENING AND INCLUDING SPACEDELECTRODE MEANS AND RADIOACTIVE MATERIAL PROVIDED IN SAID IONIZATIONCHAMBER FOR AT LEAST PARTIALLY IONIZING THE SPACE THEREBETWEEN, FILTERMEANS OPERATIVELY ASSOCIATED WITH SAID INLET OPENING OF SAID HOUSING FORPREVENTING PARTICLES ENTERING SAID IONIZATION CHAMBER WHICHSUBSTANTIALLY RESULT FROM OTHER THAN A COMBUSTION PROCESS, SAID FILTERMEANS BEING CONSTRUCTED TO PASS PARTICLES HAVING A DIAMETER SMALLER THAN1 MICRON WHILE RETAINING THE MAJORITY OF PARTICLES HAVING A DIAMETERGREATER THAN 5 MICRONS.